Inflatable evacuation slides and/or slide rafts have been installed on substantially all passenger-carrying aircraft to provide a rapid means for evacuating passengers in the event of an emergency. These inflatable devices are normally folded in an uninflated condition into a container or pack requiring a minimum of space in the interior of the aircraft and are either mounted on the interior of the aircraft door or immediately adjacent thereto. With the door closed a girt bar is connected to brackets on the floor inside the doorway such that in the event of an emergency it is only necessary to open the door to automatically deploy the slide. With the opening of the door, the girt bar will pull the slide from the door thereby allowing the slide to fall through the doorway. Subsequently the slide is rapidly inflated and is ready for evacuation of passengers within a very short period of time following opening of the door.
The descent of evacuees on an inflatable evacuation slide is primarily governed by the angle formed between the slide surface and the ground. The optimum rate is usually achieved when the angle therebetween is from about 30.degree. as a minimum to about 50.degree. as a maximum. The lower the angle, the slower the rate of descent and the evacuation may not proceed with sufficient dispatch. If the angle is much greater than 50.degree., the slide surface may be too precipitious and the evacuees will be reluctant to utilize the slide and even if they are persuaded to do so, may be injured upon impact with the ground.
Controlling the proper angle between the slide and the ground is made more complex since these evacuation slides are normally utilized only in an emergency situation. In such situations it may be that one or more of the landing gear of the aircraft have been damaged or collapsed such that the aircraft itself may be canted at a number of different possible angles. The result being that the aircraft door may be considerably higher or lower with respect to the ground than it would be in the normal situation. With the height of the door above the ground changed, the angle of the slide can be changed considerably also. Much the same situation can occur if the evacuation slide is utilized on terrain which varies in slope. This can also significantly affect the angle of the evacuation slide.
There have been a number of ideas advanced as to proper control of the speed at which an evacuee moves down a slide, both under normal operating conditions and also adapted to accommodate the extremes of landing gear collapse and/or widely varying terrain. One of the first methods advocated to accommodate the variable height of the aircraft door above the ground was to design evacuation slides of variable length. Examples of this approach are shown in U.S. Pat. Nos. 2,936,056; 3,463,266; and 3,554,344. Another early attempt at solving this problem was to provide a hinge or break in the slide such that the upper portion of the slide was at a steeper angle than the lower portion of the slide. Examples of this teaching are found in British Pat. No. 1,206,936 and U.S. Pat. Nos. 3,070,203; 3,092,232; 3,463,266, and 3,470,991. An example of a curved evacuation slide designed to accomplish the same purpose is shown in U.S. Pat. No. 3,712,417.
It should be noted that U.S. Pat. No. 2,936,056 mentioned above as an example of a variable length evacuation slide also indicates that a better sliding characteristic material may be bonded to the upper surface of the evacuation slide to improve the sliding characteristics thereof. U.S. Pat. No. 3,679,025 advocates coating the top slide surface to reduce the friction thereof. Also note U.S. Pat. No. 3,473,641 wherein the upper sliding surface is stated to have a lower coefficient of sliding friction while the lower sliding surface should have a higher sliding coefficient of friction. One additional method of slowing the descent is to be found in U.S. Pat. Nos. 3,070,203 and 3,092,232 which provide for an increased sag in the sliding surface at the lower portion of the evacuation slide. This increase sag will result in a greater coefficient of friction for the evacuee at the lower portion of the slide.